The infection of infants with AIDS generally occurs ante-or intra- partum. The logical approach to the prevention or reduction in the severity of infection is to treat the infected mother. The therapeutic agents that are currently under consideration for such use are nucleosides. It would be highly desirable to know the potential of exposure of the fetus to these agents while treating the mother. It is proposed to study in a perfusion system of human placenta the transfer and metabolism of AZT and dideoxyadenosine. Thymidine and adenosine, from which these and possible future drugs are derived, will also be studied. Efficient nucleoside transport systems have been described in cells from a variety of sources. Nothing is known about the human placenta.